GitLab

This involves introducing the concept of opt-in jobs, controlled by the
ENABLE_JOBS environment variable, analogous to the pre-existing
SUPPRESS_JOBS variable.

A large bulk of components don't currently build with the GNU tools, due to
deficiencies in the shared makefiles and the fact that we don't currently
autobuild ELF versions of libraries for them to link against. Having
softload_gnu be off by default saves us from having to explicitly disable it
in dozens of .gitlab-ci.yml files only to have to come back later to
re-enable it once the build infrastructure becomes available.

The GitLab runner machine previously ran Ubuntu 18.04, which featured
cppcheck 1.82. Some planned enhancements to the CI scripts required a newer
version of cppcheck, so we have upgraded it to Ubuntu 20.04, which has
cppcheck 1.90. However, the format of the diagnostics printed by cppcheck
has changed in 1.90, so our code that parsed them needs adapting to match.

In other words, permit files (previous false positives primarily expected to
be BASIC ones) within the Resources subdirectory of each component to lack a
copyright/licence notice.

Simplify the way we similarly omit the .git subdirectory from the test
whilst we're at it.

The exceptions list is processed into a form that excludes the given files or
directories from the `find` command that searches for files to scan. However,
a missing `-print` option caused the files to be included anyway (due to the
default action of the `find` command).

This job scans all the source files in a project, attempting to ensure they
each have a header identifying the licence terms under which the file may be
distributed. The job prints a list of the files that fail its check to the
job log, and exits with failure status if the list is non-empty.

There is no standard format for licence headers, however one thing they do
have in common is that they all feature a copyright notice. Upon scanning
all the source files hosted by ROOL, very few files failed this test, and
those that did either genuinely lacked licence terms, or had unusual ones
where the author had waived copyright and placed the code into the public
domain.

A few files used hard spaces around the word "Copyright" so the test uses
`grep -w` which considers hard spaces to constitute a word boundary.

Although not (yet?) in use by any of our sources, the ISO SPDX standard
advocates using `SPDX-FileCopyrightText` in source files in place of a
copyright declaration. Therefore, this patch pre-emptively permits this as
an alternative.

To cater for waived-copyright cases, a `.gitlab-ci.yml` file can define
`COPYRIGHT_WHITELIST` to specify files for which this test should be skipped.
This follows the same file naming pattern as `WHITESPACE_WHITELIST`.

Sometimes, a job isn't expected to pass (most often `softload_gnu`, because
certain components don't support non-Norcroft toolchains). By defining this
variable in a `.gitlab-ci.yml` file, we can force any of the jobs to be
skipped. This only applies to submodule pipelines: it is not sensible to
skip a build phase for a superproject pipeline.

We're still some way out from having these jobs pass - estimated at least 70
MRs until ROMs cross-compile. In the meantime, the main effect of these jobs
will be to cause the overall pipeline status to be amber (i.e. passed with
warnings) which undermines its usefulness as an at-a-glance indicator of
whether the other jobs all passed.

In theory, their job logs could be inspected by human reviewers to check
that the MR doesn't make a cross-compile build any worse than it already is,
but I'm not sure that this is going to be the case in practice, or whether
this overrides the benefit of being able to quickly see that the whole
pipeline passes.

So, comment out the lines that generate that part of the YAML for the time
being. Reinstate them once the respective cross-compile builds can complete.

Now that we've had chance to see how this job functions with real world MRs,
rather than just with artificial test cases, it's apparent that it is far too
sensitive and generates far too many false positives to be useful as it
currently stands. Problems include:

* When two nearby groups of lines are swapped over, the underlying `-b` and
  non-`-b` `git diff` commands can make different decisions about which group
  is fixed and which was removed from one place and inserted into another.
  When our job script finds those lines unchanged in the `-b` diff and
  changed in the non-`-b` diff, it incorrectly assumes the presence of a
  whitespace change.

* Even minor indentation changes get flagged up as an error. Even something
  as simple as changing
    bar();
  to
    if (foo)
      bar();

* In order to pass `head_whitesp` on a component that hasn't been touched
  recently, it's necessary to strip the trailing whitespace from the
  `VersionNum` file even though once the MR is accepted, this will happen
  automatically. Yet doing this gets picked up as a whitespace error change
  by `merge_whitesp`, so it's impossible to simultaneously pass all jobs!

We could just throw away this job, but there is still an important case that
ideally we do want to catch: where a source file was originally imported from
an upstream project which includes whitespace errors - meaning that we want
to preserve them to keep tracking the upstream project easy - and when one of
our contributors has accidentally stripped all trailing whitespace from the
file. We want this to be caught at the review stage - better still by the
contributor, before any reviewers have to get involved. And a CI job can
still serve this purpose well.

To distinguish these different cases, we can take into account the fact that
the accidental space-stripping scenario will typically alter many lines a
long way away from where the new contributor was working. Thus, the job
script is tightened as follows:

* Lines "nearby" a diff `-b` hunk are discounted, where "nearby" is
  arbitrarily defined as within +/- 20 lines.
* At least one line in the "removed" half of the non-`-b` hunk must have
  contained a whitespace error. It's impossible to exactly match up the
  individual "removed" and "added" lines in the general case, because there
  are often different numbers of lines in each half of a hunk.
* A threshold number of flagged lines (currently 10) must be detected across
  the whole project before the test is considered to have failed, although we
  still print any lines that we find.
* The test for introduction of whitespace errors is removed, since this
  duplicates the functionality of the `head_whitesp` job.

When working on an unrelated issue, it became clear that the reason why
previously only the `merge_log` and `merge_whitesp` jobs appeared in
detached pipelines (the ones that relate to an open MR) was really because
they include a `rules` section. It is as though in the absence of a `rules`
section, a default one applies, which adds the job to the pipeline only if
that pipeline was due to an update of a branch or tag ref. When `rules` was
present for a job, it overrides the default, and is evaluated irrespective
of the the pipeline trigger.

Now, it's useful to have all the jobs present in detached pipelines. The
latest detached pipeline state, and its associated artifactes, are displayed
at the top of the "Overview" tab of each MR page, and a new one can be
easily triggered from the "Run pipeline" button at the top of the MR page's
"Pipelines" tab, without having to navigate to the contributor's fork project
(which may not even be public). It's also a problem that the `cleanup` job
wasn't being run for detached pipelines, since over time that risks filling
up the disc space on the runner machine.

It turns out all of this can be implemented simply by adding a `workflow`
section which defines a new default set of rules, which adds jobs to
detached pipelines as well as ones for branches and tags.

Unlike all other components with CrossCompilationSupport branches, HostFS had
two, and the correct one needed to be selected in place of the HAL branch.
Now that its MRs have been merged, this can be removed.

From RiscOS/Env!15, we no longer use aliases, so there's no need to tell
non-interactive shells to expand them.

Various CI jobs run make on all the components within a project. The
COMPONENT and TARGET were correctly set, but we neglected to change into
the appropriate subdirectory first (where applicable).

These jobs were trying to fetch a build tree from a non-existent superproject
`Products/IOMD32` when they should have referenced `Products/IOMDHAL`.

On the Runner machine, each fork of each project gets its own directory,
which is left in the state which the latest job for whatever pipeline was
most recently run on it. This typically will include a large number of
object and binary files, which are of no use to anyone (anything of interest
will already have been packaged up into an artifact and uploaded to the
main GitLab server). Address this by adding an additional job to the end of
each pipeline, which does a `git clean` (it's worth leaving these in place
to reduce the bandwidth requirement when doing a `git fetch` when a pipeline
is next run for the fork).

The Runner machine also stores cache files for each fork of each project, at
least for jobs that complete fully successfully (and there are an increasing
number of these). The way our pipelines use caches, these are tarballs of
pre-built source trees for each target platform. These take up less space
than the temporary files noted above, but will now become the dominant user
of disc space. To address this, abandon use of GitLab Runner's own cache
facility, and take advantage of the fact that shell executors actually have
visibility of the gitlab-runner user's whole home directory to maintain a
single, cached version of each tarball, shared across all forks of all
projects. This is stored within ~/cache, but namespaced under ~/cache/common
to avoid collisions with any users of GitLab Runner's cache facility.

Once the following have been merged:
* Products/BCM2835!6
* Products/BuildHost!2
* Products/Disc!6
* Products/iMx6!2
* Products/IOMDHAL!3
* Products/OMAP3!3
* Products/OMAP4!3
* Products/OMAP5!3
* Products/Titanium!3
* Products/Tungsten!3
then we can have the pipelines for submodules fetch their source trees from
the central projects rather than my forks.

For these versions, "git submodule update --remote" will fail for any
submodules that don't specify a branch in .gitmodules, and which are
currently checked out on a remote tracking branch that tracks a remote
other than "origin". For our pipelines, this means almost any submodule
which has a CrossCompilationSupport branch which has not yet been merged.

Work around it by first doing "git submodule update" without "--remote",
which will put all submodules into detacehd HEAD state.

Resolves bug introduced in commit 9c61e2f0.

If the following happens:
* a GitLab runner recursively clones a superproject
* one or more submodules has changes upstream and the submodule references in
  the superproject have been updated to point to it
* a new pipeline is launched for the new superproject revision

then, while the runner would fetch changes to the superproject, it wasn't
doing so for the submodules. Ironically, we do both `git submodule update`
and `git submodule update --remote` in different pipeline stages and the
latter includes an implicit submodule fetch - but that was the later stage
which we don't get as far as.

To fix this, while retaining the pipeline stage order, include an explicit
fetch in the earlier stage (and remove the implicit fetch in the later one,
since that now merely wastes time).

This one is a special case where the superproject name mismatches the
corresponding Env (and thus Components) file.

Our own CI script also didn't previously support the *removal* of an
autogenerated YAML file. This should be a rare occurrence, but it's best
to automate this also. Also support force-pushing to the submodule; this
will be required in most cases where the pipeline was triggered by a
force-push to *this* project.